A see-through, mixed reality display device system enables a user to observe digital information overlaid on the physical scenery. To enable hands-free user interaction, a see-through, mixed reality display device system may further be equipped with an eye tracker. Typically, an eye tracker includes an infrared (IR) light source to illuminate the user's eye and a sensor (e.g., camera) to image the user's eye, e.g., to observe the reflected glints and iris movements for calculation of a gaze direction. The illumination and the imaging of the eye are preferably implemented such that: the see-through properties of the mixed reality display device system are not impaired by the eye tracking hardware; imaging of the eye works with all types of prescription spectacles; and imaging of the eye covers the entire eye movement range plus an inter-pupillary distance range and an eye relief distance range.
One way to image an eye for eye tracking is using a simple camera mounted on the frame of a head mounted display (HMD) device, wherein the camera is directly focused on the user's eye. In other words, there is a direct line of sight from the camera to the eye. While such a configuration is relatively simple and inexpensive, it is highly sensitive to the position and movement of the camera relative to the eye. Also, with such a configuration the camera needs to be positioned close to the eye level, which typically causes at least partial obstruction of the see-through properties of the mixed reality display device system. Alternatively, a partial reflector may be used to fold the camera view path to the user's temple. While this alternative configuration allows the camera to be positioned outside the see-through field, implementation of this alternative configuration is problematic if the eye tracking needs to work with prescription eyewear.
Another possibility is to use a reverse optical path imaging in a free form prism or other eyepiece based mixed reality display device system. This technique relies on the actual display optics to also provide the imaging functionality for eye tracking. However, because components of a free form prism or eyepiece tend to be rather large in size, this approach is not always practical. Adding a free form optical element for eye tracking only is also possible, but this would be expensive and would add significant weight and size to the system.